1. Field of the Invention
The present invention relates to an optical fiber transmission system for use in optical fiber communication between a main computer and its peripherals or subcomputers for household electric appliances, information apparatuses, production facilities, and the like.
2. Description of the Prior Art
Conventionally, as an optical fiber transmission system of the above-mentioned type, there has been a system provided with a single light emission section 37 and a plurality of photodetection sections 30, 31, and 32 as shown in FIG. 1, where the photodetection sections 30, 31, and 32 are connected to an optical fiber transmission line 33 via optical branch devices 34, 35, and 36 (Conventional Example 1).
In the above-mentioned conventional example, the optical branch devices 34, 35, and 36 have respective branching ratios designed to increase as the devices are closer to the light emission section 37 in order to transmit light emitted from the light emission section 37 evenly to the photodetection sections 30, 31, and 32. Therefore, disadvantageously, the larger the number of the photodetection sections is, the less the quantity of light received by each photodetection section results. Furthermore, the optical fiber transmission system of Conventional Example 1, has a low Optical coupling efficiency, and therefore the system is accompanied by a drawback that a lot of optical fiber transmission units can be hardly connected and used together.
As another conventional example, there is a system as disclosed in Japanese Patent Laid-Open Publication No. SHO 56-7540 (Conventional Example 2), which is shown in FIG. 2. This conventional example has optical fiber transmission lines 43, 44, 45, and 46 extending from a light emission section 47; optical filters 48, 49, and 50 provided between the optical fiber transmission lines 43 through 46; and photodetection sections 40, 41, and 42 provided for receiving light from the optical filters 48 through 50 via optical fibers 51, 52, and 53, respectively.
The above-mentioned light emission section 47 is comprised of a semiconductor laser device of which operation characteristics are shown in FIG. 3. As shown in FIG. 3, the semiconductor laser device emits laser light having discontinuous center wavelengths of .lambda..sub.1, .lambda..sub.2, and .lambda..sub.3 according as an operation temperature or a drive current is changed.
The optical filters 48, 49, and 50 have their respective characteristics 54, 55, and 56, which are shown in FIG. 4, and extract light at the wavelengths of .lambda..sub.1, .lambda..sub.2, and .lambda..sub.3 from the aforementioned optical fiber transmission lines. Then the light from the optical filters 48, 49, and 50 reach the photodetection sections 40, 41, and 42 via the optical fibers 51, 52, and 53, respectively.
Therefore, when the semiconductor laser device is made to emit light having the wavelength of, for example, .lambda..sub.2 by changing its drive current, the light passes through the optical filter 48 without any substantial loss, and reaches the optical filter 49 via the optical fiber transmission line 44. Then most of the light having the wavelength of .lambda..sub.2 is reflected on the optical filter 49, and reaches the photodetection section 41 via the optical fiber 52.
Therefore, in conventional Example 2, (1) because an optical signal from the light emission section 47 is not transmitted to any photodetection section other than the objective photodetection section, the quantity of light received by the objective photodetection section is greater than that of Conventional Example 1. Therefore, (2) optical transmission over a long distance can be achieved, and moreover, a lot of photodetection sections can be provided on the optical fiber transmission line. Furthermore, (3) by changing the wavelength of light emitted from the light emission section, the destination photodetection section to which an optical signal is desired to be transmitted can be easily selected.
The optical fiber transmission system of Conventional Example 2 can obtain an improved optical coupling efficiency. However, in order to specify a destination photodetection section which is the other party of the optical fiber transmission, it is required to constitute the light emission section 47 by a semiconductor laser and drive current changing means and interpose the optical filters 48, 49, and 50 between the optical fiber transmission lines 43, 44, 45, and 46. This arrangement is accompanied by a problem that a complicated optical transmission structure results and therefore that the costs increase.
Furthermore, the semiconductor laser is required to have the same number of center wavelengths as the number of the photodetection sections. Accordingly, there is a problem that there is a limitation in number of the receiving sections.
Next, FIG. 5 shows Conventional Example 3 which is disclosed in Japanese Patent Laid-Open Publication No. SHO 56-10748. This conventional example has a first optical transmitter-receiver 91, a second optical transmitter-receiver 92, and an optical fiber 72. The first optical transmitter-receiver 91 has an optical transmitter 61, an optical receiver 74, an optical filter 63, and a two-way separating circuit 71. The second optical transmitter-receiver 92 has an optical transmitter 62, an optical receiver 75, an optical filter 64, and a two-way separating circuit 73. The optical filter 63 has a light transmittance characteristic T1 which is shown in FIG. 6, while the optical filter 64 has a light transmittance characteristic T2 as shown in FIG. 6.
The optical transmitters 61 and 62 have electric input terminals 69 and 70, drive circuits 65 and 66, and light emitting diodes 67 and 68, respectively. An electric signal inputted from each of the electric input terminals is amplified in the drive circuits, and then inputted to the light emitting diodes to be converted into an optical signal by the light emitting diodes. The light emitting diodes 67 and 68 have different emission light center wavelengths. In detail, as shown in FIG. 6, the light emitting diode 67 has an emission spectrum P1 having an emission light center wavelength of about 830 nm and an emission spectrum full-width at half maximum of about 45 nm. On the other hand, the light emitting diode 68 has an emission spectrum P2 having an emission light center wavelength of about 850 nm and an emission spectrum full-width at half maximum of about 45 nm. The emission spectrum P1 of the light emitting diode 67 and the emission spectrum P2 of the light emitting diode 68 are partially overlapping each other. The light emitting diodes 67 and 68 are GaAlAs light emitting diodes.
The optical receivers 74 and 75 have photodiodes 76 and 77, amplifier circuits 78 and 79, and electric output terminals 80 and 81, respectively. Light inputted to each of the photodiodes is converted into an electric signal in the photodiodes, and the resulting electric signal is amplified in the amplifier circuits and then transmitted to the electric output terminals.
The two-way separating circuit 71 transmits light from the optical transmitter 61 to the optical fiber 72, and does not transmit the light to the optical receiver 74. Further, the two-way separating circuit 71 transmits light from the optical fiber 72 to the optical receiver 74, and does not transmit the light to the optical transmitter 61. The two-way separating circuit 73 transmits light from the optical transmitter 62 to the optical fiber 72, and does not transmit the light to the optical receiver 75. Further, the two-way separating circuit 73 transmits light from the optical fiber 72 to the optical receiver 75, and does not transmit the light to the optical transmitter 62.
According to the Conventional Example 3, when an electric signal is inputted from the electric input terminal 69 of the optical transmitter 61, the electric signal is transmitted through the driving circuit 65 and then makes the light emitting diode 67 emit light. The light emitted from the light emitting diode 67 is inputted to the optical filter 63 and only an emission spectrum corresponding to T1 in FIG. 6 is allowed to pass through the filter and then inputted to the two-way separating circuit 71. Then the light is outputted from the two-way separating circuit 71 to the optical fiber 72, then to the second two-way separating circuit 73, and then to the second optical receiver 75. The light inputted to the second optical receiver is extracted in a form of an electric signal from the electric output terminal 81 by way of the photodiode 77 and the amplifier circuit 79.
In the Conventional Example 3, the center wavelength .lambda..sub.1 of the emission spectrum P1 of the first light emitting diode 67 and the center wavelength .lambda..sub.2 of the emission spectrum P2 of the second light emitting diode 68 are close to each other. However, the emission spectrum of the light which has been emitted from the first light emitting diode 67 and passed through the first optical filter 63 and the emission spectrum of the light which has been emitted from the second light emitting diode 68 and passed through the second optical filter 64 are substantially not overlapping each other. Therefore, due to the difference of the emission spectrums, the light from the first light emitting diode 67 and the light from the second light emitting diode 68 can be separated by the two-way separating circuits 71 and 73.
The optical fiber transmission system of Conventional Example 3 can also obtain an improved optical coupling efficiency. However, the optical transmitter-receivers 91 and 92 are obliged to surely separate the light transmitted from the optical transmitter-receivers to the optical fiber 72 from the light transmitted to the optical transmitter-receivers by way of the optical fiber 72. Therefore, the optical transmitter-receivers 91 and 92 are each required to have a light emitting diode exhibiting an emission spectrum different from that of the other optical transmitter-receiver, an optical filter, and a two-way separating circuit. This results in a complicated optical transmission structure, increasing the costs.